Techniques of converting an image into an image given a blurring effect has existed. For example, in one technique, to execute a process of giving a blurring effect, a blurring range is calculated for each pixel to be processed in an input image. In another technique, a filter process is executed by using multiple moving average filters of different sizes to generate multiple blurred images, and weight is given to mutually corresponding pixels of the multiple blurred image data for each of the blurred images to use an average value of the blurred image data for a composite blurred image. In a further technique, an exposure amount is obtained by a calculation from an RGB value of each pixel of an input image to execute a process of giving a blurring effect to the exposure amount of each pixel, and the exposure amount of each pixel given the blurring effect is restored to an RGB value (see, e.g., Japanese Laid-Open Patent Publication Nos. H6-036022; 2004-133551; and 2001-216513).
However, the conventional techniques described above is problematic in that the amount of calculation involved in the process of giving a blurring effect to an image becomes large and results in an increase in the processing time consumed for a process of converting an image. For example, the process of giving a blurring effect is a process of distributing values of pixels according to an aperture shape defined as a blurring range for each pixel in an image, leading to a large calculation amount.